Voice communications, in general, do not utilize the speed of the links between source and destination because of periods of silence and also because there is usually speech activity only in one direction. TASI (Time Assigned Speech Interpolation) is a technique whereby the silences in speech are employed to increase the number of voice calls that a link can carry. The basic idea behind TASI is that whenever a silence period occurs in a particular voice call, the slot allocated to that voice call is freed and assigned, if necessary, to another voice call in talkspurt, that is, a call exhibiting voice activity. The basic costs in implementing TASI are the link signalling required to reassign the slots and the speech activity detection to determine the silence periods. In current TASI systems, the link signalling is done through a separate channel between transmitter and receiver. The TASI function is performed independently over each hop, and thus the speech activity detection for each voice call is also done separately for each hop. Consequently, the cost of TASI is prohibitive and it is implemented only over expensive trans-oceanic lines.
The extension of the TASI feature to a full network, that is, over the multiple switching nodes which are typically present between the conversing parties, is of significant value. However, in addition to the aforementioned difficulties in present TASI implementations, the circuit switches associated with each node must be reconfigured sufficiently fast whenever a new path is required. Since most conventional circuit switches are not fast enough and would result in voice clipping, a multihop TASI scheme which requires such rapid reconfiguration is of limited utility.
What is desired is a system that performs TASI over every hop with a single instance of speech activity detection for each voice call. That is, voice detection is accomplished at the source and each circuit switch signals the succeeding switch until the call reaches its destination. Moreover, the signalling over each link should be accomplished in-band, that is, it should not require a separate physical channel. Finally, the system should use conventional circuit switches and should not require that these switches be reconfigured every time a new call is originated. The system of the present invention exemplifies these features.